I have the 40/52 out to a 13-30. Most of the time, I ride in the 40. Coming off a hill, I ride in the 52; but if the area stays flat, I can stay in the 52 for quite some time (not too often around here though).

The overlap can be nice feature. Could you imagine going through all 6 gears in the small chainring, then having to upshift to the big chainring and having to downshift all the way on the rear for the next gear ratio? Ie, zero overlap. That would be very annoying, let's say the terrain was alternating between those two gears. You'd be shifting both derailers constantly.

Instead, on most bikes, sure, the front chainrings is worth say 2 gears in the rear, which would mean 3 or 4 overlapping gears. But that overlap means that you don't have to always shift both derailers at the same time. Hopefully, both chainrings have a couple of overlapping gear ratios that you cruise at, that way, small terrain changes won't force a FD shift.